The present invention relates to drying of liquid basecoats for automotive coating applications and, more particularly, to multi-stage processes for drying a liquid basecoat which include a combination of infrared radiation and convection drying for subsequent powder topcoat application.
Today""s automobile bodies are treated with multiple layers of coatings which not only enhance the appearance of the automobile, but also provide protection from corrosion, chipping, ultraviolet light, acid rain and other environmental conditions which can deteriorate the coating appearance and underlying car body.
The formulations of these coatings can vary widely. However, a major challenge that faces all automotive manufacturers is how to rapidly dry and cure these coatings with minimal capital investment and floor space, which is valued at a premium in manufacturing plants.
Various ideas have been proposed to speed up drying and curing processes for automobile coatings, such as hot air convection drying. While hot air drying is rapid, a skin can form on the surface of the coating which impedes the escape of volatiles from the coating composition and causes pops, bubbles or blisters which ruin the appearance of the dried coating.
Other methods and apparatus for drying and curing a coating applied to an automobile body are disclosed in U.S. Pat. Nos. 4,771,728; 4,907,533; 4,908,231 and 4,943,447 in which the automobile body is heated with radiant heat for a time sufficient to set the coating on Class A surfaces of the body and subsequently cured with heated air.
U.S. Pat. No. 4,416,068 discloses a method and apparatus for accelerating the drying and curing of refinish coatings for automobiles using infrared radiation. Ventilation air used to protect the infrared radiators from solvent vapors is discharged as a laminar flow over the car body. FIG. 15 is a graph of temperature as a function of time showing the preferred high temperature/short drying time curve 122 versus conventional infrared drying (curve 113) and convection drying (curve 114). Such rapid, high temperature drying techniques can be undesirable because a skin can form on the surface of the coating that can cause pops, bubbles or blisters, as discussed above.
U.S. Pat. No. 4,336,279 discloses a process and apparatus for drying automobile coatings using direct radiant energy, a majority of which has a wavelength greater than 5 microns. Heated air is circulated under turbulent conditions against the back sides of the walls of the heating chamber to provide the radiant heat. Then, the heated air is circulated as a generally laminar flow along the inner sides of the walls to maintain the temperature of the walls and remove volatiles from the drying chamber. As discussed at column 7, lines 18-22, air movement is maintained at a minimum in the central portion of the inner chamber in which the automobile body is dried.
A rapid, multi-stage drying process for automobile coatings is needed which inhibits formation of surface defects and discoloration in the coating, particularly for use with liquid basecoats to be overcoated with powder topcoat.
The present invention provides a process for coating a metal substrate, comprising the steps of: (a) applying a liquid basecoating composition to a surface of the metal substrate; (b) applying a first infrared radiation at a power density of 30 kilowatts per meter squared or less and a first air simultaneously to the basecoating composition for a first period of at least about 30 seconds, a first velocity of the air at the surface of the basecoating composition being about 4 meters per second or less, a first temperature of the metal substrate being increased at a first rate ranging from about 0.05xc2x0 C. per second to about 0.3xc2x0 C. per second to achieve a first peak metal temperature ranging from about 30xc2x0 C. to about 60xc2x0 C., such that a pre-dried basecoat is formed upon the surface of the metal substrate; (c) applying a second infrared radiation and a second air simultaneously to the basecoating composition for a second period of at least about 15 seconds, a second velocity of the air at the surface of the basecoating composition being about 4 meters per second or less, a second temperature of the metal substrate being increased at a second rate ranging from about 0.4xc2x0 C. per second to about 1.2xc2x0 C. per second to achieve a second peak metal temperature of the substrate ranging from about 60xc2x0 C. to about 80xc2x0 C., such that a dried basecoat is formed upon the surface of the metal substrate; and (d) applying a powder topcoating composition over the dried basecoat.
Another aspect of the present invention is a process for coating a metal substrate, comprising the steps of: (a) applying a liquid basecoating composition to a surface of the metal substrate; (b) applying a first air to the basecoating composition for a first period of at least about 1 minute to volatilize at least a portion of volatile material from the liquid basecoating composition, the air at a surface of the basecoating composition having a first velocity that is about 0.5 meters per second or less; (c) applying a first infrared radiation at a power density of 30 kilowatts per meter squared or less and a second air simultaneously to the basecoating composition for a second period of at least about 30 seconds, a second velocity of the air at the surface of the basecoating composition being about 4 meters per second or less, a first temperature of the metal substrate being increased at a first rate ranging from about 0.05xc2x0 C. per second to about 0.3xc2x0 C. per second to achieve a first peak metal temperature ranging from about 30xc2x0 C. to about 60xc2x0 C., such that a pre-dried basecoat is formed upon the surface of the metal substrate; (d) applying a second infrared radiation and a third air simultaneously to the basecoating composition for a third period of at least about 15 seconds, a third velocity of the air at the surface of the basecoating composition being about 4 meters per second or less, a second temperature of the metal substrate being increased at a second rate ranging from about 0.4xc2x0 C. per second to about 1.2xc2x0 C. per second to achieve a second peak metal temperature of the substrate ranging from about 60xc2x0 C. to about 80xc2x0 C., such that a dried basecoat is formed upon the surface of the metal substrate; and (e) applying a powder topcoating composition over the dried basecoat.
Yet another aspect of the present invention is a process for coating a polymeric substrate, comprising the steps of: (a) applying a liquid basecoating composition to a surface of the polymeric substrate; (b) applying a first infrared radiation at a power density of 30 kilowatts per meter squared or less and a first air simultaneously to the basecoating composition for a first period of at least about 30 seconds, a first velocity of the air at the surface of the basecoating composition being about 4 meters per second or less, a first temperature of the polymeric substrate being increased at a first rate ranging from about 0.05xc2x0 C. per second to about 0.3xc2x0 C. per second to achieve a first peak polymeric temperature ranging from about 30xc2x0 C. to about 60xc2x0 C., such that a pre-dried basecoat is formed upon the surface of the polymeric substrate; (c) applying a second infrared radiation and a second air simultaneously to the basecoating composition for a second period of at least about 15 seconds, a second velocity of the air at the surface of the basecoating composition being about 4 meters per second or less, a second temperature of the polymeric substrate being increased at a second rate ranging from about 0.4xc2x0 C. per second to about 1.2xc2x0 C. per second to achieve a second peak polymeric temperature of the substrate ranging from about 60xc2x0 C. to about 80xc2x0 C., such that a dried basecoat is formed upon the surface of the polymeric substrate; and (d) applying a powder topcoating composition over the dried basecoat.
Another aspect of the present invention is a process for coating a polymeric substrate, comprising the steps of: (a) applying a liquid basecoating composition to a surface of the polymeric substrate; (b) applying a first air to the basecoating composition for a first period of at least about 1 minute to volatilize at least a portion of volatile material from the liquid basecoating composition, the air at a surface of the basecoating composition having a first velocity of about 0.5 meters per second or less; (c) applying a first infrared radiation at a power density of 30 kilowatts per meter squared or less and a second air simultaneously to the basecoating composition for a second period of at least about 30 seconds, a second velocity of the air at the surface of the basecoating composition being about 4 meters per second or less, a first temperature of the polymeric substrate being increased at a first rate ranging from about 0.05xc2x0 C. per second to about 0.3xc2x0 C. per second to achieve a first peak polymeric temperature ranging from about 30xc2x0 C. to about 60xc2x0 C., such that a pre-dried basecoat is formed upon the surface of the polymeric substrate; (d) applying a second infrared radiation and a third air simultaneously to the basecoating composition for a third period of at least about 15 seconds, a third velocity of the air at the surface of the basecoating composition being about 4 meters per second or less, a second temperature of the polymeric substrate being increased at a second rate ranging from about 0.4xc2x0 C. per second to about 1.2xc2x0 C. per second to achieve a second peak polymeric temperature of the substrate ranging from about 60xc2x0 C. to about 80xc2x0 C., such that a dried basecoat is formed upon the surface of the polymeric substrate; and (e) applying a powder topcoating composition over the dried basecoat.